Implement dynamic loading of GnuTLS on Windows.

Implement dynamic loading of GnuTLS on Windows.

* lisp/term/w32-win.el (dynamic-library-alist): Add `gnutls'.

* nt/INSTALL: Clarify GnuTLS support.

* src/callproc.c, src/emacs.c: Include lisp.h before src/w32.h, not after.

* src/gnutls.c (Qgnutls_dll): Define.
  (DEF_GNUTLS_FN, LOAD_GNUTLS_FN): New macros.
  (gnutls_*): Declare function pointers.
  (init_gnutls_functions): New function to initialize function pointers.
  (emacs_gnutls_handshake, Fgnutls_error_string, Fgnutls_deinit)
  (emacs_gnutls_global_init, Fgnutls_bye): Use function pointers.
  (emacs_gnutls_record_check_pending, emacs_gnutls_transport_set_errno):
  Wrappers for gnutls_record_check_pending and gnutls_transport_set_errno.
  (emacs_gnutls_write, emacs_gnutls_read)
  (emacs_gnutls_handle_error, Fgnutls_error_fatalp)
  (Fgnutls_available_p): New function.
  (Fgnutls_boot): Call Fgnutls_available_p.  Use function pointers.
  (syms_of_gnutls) <Qgnutls_dll>: Initialize and staticpro it.
  (syms_of_gnutls) <Sgnutls_available_p>: defsubr it.

* src/gnutls.h (GNUTLS_EMACS_ERROR_NOT_LOADED): New macro.
  (emacs_gnutls_write, emacs_gnutls_read): Mark as extern.
  (emacs_gnutls_record_check_pending, emacs_gnutls_transport_set_errno):
  Declare.

* src/w32.c (QCloaded_from, Vlibrary_cache): Define.
  (w32_delayed_load): Move from image.c.  When loading a library, record
  its filename in the :loaded-from property of the library id.
  (globals_of_w32) <QCloaded_from, Vlibrary_cache>:
  Initialize and staticpro them.
  (emacs_gnutls_pull, emacs_gnutls_push): Call emacs_gnutls_* functions.

* src/image.c: Include w32.h.
  (Vimage_type_cache): Delete.
  (syms_of_image) <Vimage_type_cache>: Don't initialize and staticpro it.
  (CACHE_IMAGE_TYPE, Finit_image_library): Use Vlibrary_cache instead.
  (w32_delayed_load): Move to w32.c.

* src/process.c: Include lisp.h before src/w32.h, not after.
  (wait_reading_process_output): Call emacs_gnutls_record_check_pending
  instead of gnutls_record_check_pending.

* src/w32.h (VlibraryCache, QCloaded_from, w32_delayed_load): Declare.